Invar forming method for making tooling

ABSTRACT

A method for fabricating an Invar tool includes the step of superplastically forming an Invar facesheet for the tool. A tool header is provided, and the Invar facesheet is then formed with an SPF diaphragm driver to match the predetermined contour of the tool header. The Invar facesheet is then welded to the tool header. Forming the Invar facesheet is achieved by positioning the Invar facesheet between a superplastically formable diaphragm and a die having the predetermined contour. The diaphragm is heated to a predetermined superplastic temperature, and a predetermined pressure is applied to the diaphragm. Application of the predetermined pressure allows the diaphragm to cause the Invar facesheet to form against the predetermined contour of the die. Superplastically forming the Invar facesheet results in a negligible amount of excess scrap, reduces material costs, and reduces labor costs.

TECHNICAL FIELD

The present invention relates generally to Invar tool fabrication. Moreparticularly, the invention relates to a method for fabricating an Invartool including the step of superplastically forming an Invar facesheetto match or to approximate a predetermined contour of the tool.

BACKGROUND OF THE INVENTION BACKGROUND ART

Modern day aerospace, defense, and high tech research applicationstypically require complex, high precision, and close tolerance tooling.Thermal expansion is an area of particular concern in the aerospaceindustry with regard to tooling. For example, most military aircraftapplications stipulate that tools exhibit minimal dimensional changeswhen subjected to extreme temperature variations.

In the campaign to control thermal expansion, a useful material has beendeveloped under the tradename of Invar 36. Invar 36 (“Invar”) is anickel-iron alloy that has a rate of thermal expansion that isapproximately one-tenth that of carbon steel at temperatures up to 400degrees F. Invar therefore has a particular attractiveness infabricating tool components, including a header, which serves as a“frame” for a tool, and a facesheet, which is welded to the header.

Several of our first approaches in developing the present invention werelabor and material intensive. One such approach involved a stretchforming technique as shown in FIGS. 3 and 4. Specifically, a stretchform die was fabricated to a predetermined contour, and the Invarfacesheet was stretched over the die surface. We considered this formingprocess to be unsuccessful as it required a substantial amount of excessInvar material for the jaws of the stretch press which are used to gripthe facesheet. As the amount of this excess Invar material was typicallyequal to the amount of material that was required to make the facesheet,and as the excess material could not be used as the part of the tool,the excess Invar material needed to be trimmed, which led to substantialwaste. Another one of our approaches involved bump forming thick Invarmaterial, and cutting the material to fit the tool in segments. Yetanother one of our approaches included the use of castings. Thisapproach, however, was very expensive due to the need to eliminateporosity. It is therefore desirable to provide a method and assembly forfabricating an Invar tool with reduced labor and material costs.

SUMMARY OF THE INVENTION

A preferred method of the present invention for fabricating an Invartool includes the step of providing a tool header defining apredetermined contour. An Invar facesheet is then superplasticallyformed to match, accurately or substantially, the predetermined contourof the tool header. Normally, the method further provides for weldingthe Invar facesheet to the tool header. Forming the Invar facesheet inthis way results in less scrap, and therefore substantially reducesmaterial costs. Forming the Invar facesheet in this way also leads toreduced labor costs.

The present invention further provides an Invar tool. The Invar toolincludes a tool header defining a predetermined contour, and one or moreInvar facesheets welded to the tool header. The Invar facesheets areformed with an SPF diaphragm driver to substantially match thepredetermined contour of the tool header.

Further in accordance with the present invention, a method for moldingparts on Invar tooling is provided. The method includes the step ofmaking an Invar tooling surface from a sheet of Invar with SPF diaphragmforming. The sheet is placed on a tool header, and a part is formedagainst or on the tooling surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art by reading the following specification andsub-joined claims and by referencing the following drawings in which:

FIG. 1 is a perspective view of an Invar tool fabricated in accordancewith the present invention;

FIG. 2 is a perspective view of a superplastically formed Invarfacesheet in accordance with the present invention;

FIG. 3 is a perspective view of an Invar tool fabricated in accordancewith one of our first approaches in developing the present invention;

FIG. 4 is a perspective view of an Invar facesheet formed in accordancewith one of our first approaches in developing the present invention;

FIG. 5 is an exploded perspective view of a superplastic formingassembly in accordance with the present invention;

FIG. 6a is a cross-sectional side view taken along lines 6—6 in FIG. 5of a superplastic forming assembly before fabrication in accordance withthe present invention;

FIG. 6b is a cross-sectional side view taken along lines 6—6 in FIG. 5of a superplastic forming assembly during fabrication in accordance withthe present invention;

FIG. 6c is a cross-sectional side view taken along lines 6—6 in FIG. 5of a superplastic forming assembly after fabrication in accordance withthe present invention;

FIG. 7 is an enlarged side view of area A shown in FIG. 6c;

FIG. 8 is a cross-sectional side view taken along lines 8—8 in FIG. 5 ofa superplastic forming assembly in accordance with the presentinvention;

FIG. 9 is a perspective view of a mold for a ceramic die in accordancewith the present invention;

FIG. 10 is a side view of a process for pouring a ceramic castablematerial into the mold shown in FIG. 9; and

FIG. 11 is a perspective view of a ceramic die loaded onto a press bedin accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, an Invar tool 20 fabricated in accordancewith the principles of the present invention includes a tool header 21having a predetermined contour and an Invar facesheet 22. Invarfacesheets 22 are preferably superplastically formed in a diaphragmforming process using an SPF alloy driver to match the predeterminedfinal contour in segments. The segments are welded to the tool header 21to define the forming surface of the tool and to complete the tool withminimal waste of the Invar.

FIG. 2 shows the Invar facesheet 22 in greater detail and demonstratesthe advantages over conventional approaches. To more fully demonstratethe improvements provided by the present invention, FIGS. 3 and 4 show aconventional Invar tool 30 and conventional Invar facesheet 31 inaccordance with previous manufacturing approaches. Specifically, it willbe appreciated that the formed Invar facesheet 22 of the presentinvention is able to meet more complex contours than a conventionalfacesheet 31. As a result, fewer are needed and labor intensive weldingis reduced. For example, seven sections of conventional facesheet 31must be fabricated for the tool header 21, whereas only twosuperplastically formed Invar facesheets 22 are required for the sametool header 21. It will further be appreciated that the conventionalfacesheet 31 results in substantial waste to trim edges from the sheetsfollowing the stretch forming process.

FIG. 5 shows a preferred superplastic forming assembly 40 of the presentinvention. The preferred superplastic forming (SPF) assembly 40 includesa press bed 41, and a die 42 coupled to the press bed 41. The die 42 hasa predetermined contour corresponding to the Invar tool 20 (FIG. 1). TheSPF assembly 40 further includes a superplastically formable diaphragm43, and a sealing ring 44 positioned between the die 42 and thediaphragm 43. A press head 45 is coupled to the diaphragm 43. The presshead 45 has an associated head weight, and applies a predeterminedpressure to the diaphragm 43 such that the Invar facesheet 22 formsagainst the predetermined contour of the die 42. It will be appreciatedthat the head weight is sufficient to offset the reactive force of thepredetermined pressure. Further details of a suitable SPF press aredescribed in U.S. Pat. No. 5,467,626.

FIGS. 6a-6 c demonstrate the process of forming the Invar facesheet 22in greater detail. Specifically, the Invar facesheet 22 is positionedbetween the diaphragm 43 and the die 42 having the predetermined contour46. The diaphragm 43 is a superplastic alloy, such as titanium. It isheated to a predetermined superplastic temperature, and a predeterminedpressure is applied to the diaphragm 43 such that the diaphragm 43causes the Invar facesheet 22 to form against the predetermined contour46 of the die 42. The Invar facesheet softens at the SPF temperature,allowing it to be stretched with the diaphragm. In one embodiment, thediaphragm 43 includes titanium, and the predetermined pressure isapplied via argon gas. Superplastically forming the Invar facesheet 22allows the Invar facesheet 22 to be profiled to accommodate thepredetermined contour 46 before forming and results in a substantialreduction in material waste. An important aspect of the SPF processinvolves proper forming of the die 42 to allow unrestricted fabricationof the Invar facesheet 22.

Thus, the Invar facesheet 22 transitions from a flat shape as shown inFIG. 6a, to a partially formed shape as shown in FIG. 6b, to a finallyformed shape as shown in FIG. 6c. It is also important to note that asshown in FIG. 8, the Invar facesheet 22 can be formed in multipledirections as dictated by the predetermined contour 46. This increasedability to customize the Invar facesheet is highly desirable overconventional stretch forming approaches to Invar tool fabrication.

FIG. 7 shows that the sealing ring 44 undergoes compression in responseto the head weight of the press head 45. The sealing ring 44 ispreferably made of stainless steel and adds stability to the overall SPFprocess.

It is preferred that the die 42 be formed from a ceramic castablematerial, as described in U.S. Pat. No. 5,467,626. Thus, as shown inFIG. 9, the process of forming the die 42 includes the step of creatinga mold 50 having the predetermined contour 46. FIG. 10 demonstrates thatthe ceramic castable material 51 is poured into the mold 50. The ceramiccastable material 51 is then allowed to set into the ceramic die 42, andthe ceramic die 42 is demolded. As shown in FIG. 11, the ceramic die 42is cured at the predetermined superplastic temperature, and loaded ontothe press bed 41. In one embodiment, the predetermined superplastictemperature is approximately 1650 degrees F., and the predeterminedpressure of the argon gas is approximately 90-180 psi. Curing theceramic die 42 at the predetermined superplastic temperature insuresthat the ceramic die 42 will not undergo undesirable thermal expansionduring the SPF process.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and the following claims.

What is claimed is:
 1. A method for fabricating an Invar tool having atooling surface, the method comprising the steps of: providing a toolheader defining a predetermined concave contour; forming an Invarfacesheet with an SPF diaphragm driver in a superplastic formingoperation such that a first side of the Invar facesheet substantiallyconforms to the predetermined concave contour of the tool header and asecond side of the Invar facesheet substantially conforms to a convexcontour of the tooling surface; and attaching the Invar facesheet to thetool header such that the attached Invar facesheet defines at least aportion of the tooling surface.
 2. The method of claim 1 furtherincluding the steps of: positioning the Invar facesheet between asuperplastically formable diaphragm and a die having the predeterminedcontour; heating the diaphragm and the facesheet to a predeterminedsuperplastic temperature; and applying a predetermined pressure to thediaphragm such that the diaphragm forms into the die superplasticallyand causes the Invar facesheet to form against the predetermined contourof the die.
 3. The method of claim 2 further including the step ofprofiling the Invar facesheet to accommodate the predetermined contour.4. The method of claim 2 further including the step of forming the dieto allow unrestricted forming of the Invar facesheet.
 5. The method ofclaim 4 further including the step of forming the die from a ceramiccastable material.
 6. The method of claim 5 further including the stepsof: creating a mold having the predetermined contour; pouring theceramic castable material into the mold; allowing the ceramic castablematerial to set into a ceramic die; demolding the ceramic die; curingthe ceramic die at the predetermined superplastic temperature; andloading the ceramic die onto a press bed.
 7. The method of claim 2further including the steps of: positioning a sealing ring between thediaphragm and the die; and positioning the sealing ring and thediaphragm between the die and a press head having an associated headweight, the press head applying the predetermined pressure to thediaphragm and the head weight being sufficient to offset thepredetermined pressure.
 8. The method of claim 2 wherein thepredetermined superplastic temperature is approximately 1650 degrees F.9. The method of claim 2 further including the step of applying argongas to the diaphragm.
 10. The method of claim 9 wherein thepredetermined pressure of the argon gas is approximately 90-180 psi. 11.The method of claim 1, further comprising the steps of: forming a secondInvar facesheet with one of the SPF diaphragm driver and a second SPFdiaphragm driver such that a first side of the second Invar facesheetsubstantially conforms to the predetermined contour of the tool headerand a second side of the second Invar facesheet substantially conformsto the contour of the tooling surface; and welding the second Invarfacesheet to the tool header and the Invar facesheet such that thesecond Invar facesheet defines a second portion of the tooling surface.12. A method for forming an Invar facesheet for an Invar tool having apredetermined contour, the method comprising the steps of: positioningthe Invar facesheet between a superplastically formable diaphragm and adie having the predetermined contour; heating the diaphragm to apredetermined superplastic temperature; and applying a predeterminedpressure to the diaphragm such that the diaphragm forms superplasticallyinto the die and causes the Invar facesheet to form against thepredetermined contour of the die wherein the Invar facesheet is formedto include a first side having a convex shape that is configured toconform to a predetermined tooling surface and a second side having aconcave shape that is configured to conform to a surface of a toolheader.
 13. The method of claim 12 further including the step ofprofiling the Invar facesheet to accommodate the predetermined contour.14. The method of claim 12 further including the step of forming the dieto allow unrestricted forming of the Invar facesheet.
 15. The method ofclaim 14 further including the step of forming the die from a ceramiccastable material.
 16. The method of claim 15 further including thesteps of: creating a mold having the predetermined contour; pouring theceramic castable material into the mold; allowing the ceramic castablematerial to set into a ceramic die; demolding the ceramic die; curingthe ceramic die at the predetermined superplastic temperature; andloading the ceramic die onto a press bed.
 17. The method of claim 15wherein the ring includes stainless steel.
 18. The method of claim 12further including the steps of: positioning a sealing ring between thediaphragm and the die; and positioning the sealing ring and thediaphragm between the die and a press head having an associated headweight, the press head applying the predetermined pressure to thediaphragm and the head weight being sufficient to offset thepredetermined pressure.
 19. The method of claim 12 wherein the diaphragmincludes titanium.